The present invention generally relates to production of nitride crystals and more particularly to a crystal growth apparatus used to grow a crystal of group III nitride and a method of producing such a group III nitride crystal.
These days, most of the InGaAlN (a group III nitride semiconductor) devices used for ultraviolet, purple, blue and green optical sources are formed on a substrate of sapphire or silicon carbide (SiC) by conducting thereon an MOCVD process (metal-organic chemical vapor deposition process) or MBE process (molecular beam epitaxy process).
In the case a sapphire or silicon carbide is used for the substrate, however, there are formed a large number of crystal defects in the group III nitride semiconductor layers grown thereon in view of the fact that there exists a large difference in the thermal expansion coefficient and lattice constant between the substrate and the group III nitride semiconductor layers, and because of such crystal defects, there is caused degradation in the device performance. In the case of a light-emitting device, for example, problems such as short lifetime, large operational power consumption, and the like, are caused as a direct consequence of such crystal defects existing with high density.
Further, because a sapphire substrate is an insulator, it is impossible to provide an electrode directly on the substrate contrary to conventional light-emitting devices constructed on a semiconductor substrate. This means that it is necessary to provide an electrode on one of the group III nitride semiconductor layers, while such a construction necessitates large device area for formation of the electrode and the cost of the device is increased inevitably. In addition, there is caused a problem of warp of the substrate because of the use of different materials such as sapphire substrate in combination with the group III nitride semiconductor layers. This problem of warp becomes a serious problem particularly when the device area is increased.
Further, with the group III nitride semiconductor device constructed on a sapphire substrate, chip separation by way of cleaving process is difficult, and it is not easy to obtain an optical cavity edge surface, which is required in laser diodes (LD). Because of this, it has been practiced in the art, when to form an optical cavity edge surface, to conduct a separation process similar to a cleaving process after reducing the thickness of the sapphire substrate to 100 μm or less by conducting a dry etching process or polishing process.
Thus, it has been difficult to conduct formation of optical cavity edge surface and chip separation of with a single step, contrary to the production process of conventional laser diodes, and there has been a problem of increased cost because of the complexity of the fabrication process of light-emitting devices.
In order to solve these problems, there has been made a proposal of reducing the crystal defects by conducting selective growth process of the group III nitride semiconductor layers on the sapphire substrate in a lateral direction. With this approach, it has become possible to reduce the crystal defects successfully, while there still remain problems of insulating nature of the sapphire substrate and difficulty of cleaving a sapphire substrate with such a construction.
In order to solve these problems, use of a gallium nitride (GaN) substrate of generally the same composition to the crystalline materials grown thereon is preferable. Thus, various attempts have been made for growing a bulk GaN crystal by vapor phase growth process or melt growth process. However, GaN substrate of high quality and practical size is not yet realized.
As one approach of realizing a GaN bulk crystal substrate, there-is proposed a GaN crystal growth process that uses sodium (Na) for the flux (Patent Reference 1). According to this method, sodium azide (NaN3) and metal Ga are confined in a reaction vessel of stainless steel (vessel dimension: inner diameter=7.5 mm; length=100 mm) as the source material, together with a nitrogen gas, and a GaN crystal is grown by holding the reaction vessel at a temperature of 600-800° C. for 24-100 hours.
According to this method, it becomes possible to carry out the crystal growth at relatively low temperatures of 600-800° C. while maintaining the pressure inside the vessel to a relatively low pressure of 100 kg/cm2 or less. This means that crystal growth can be conducted under a practical condition.
Further, there is realized a high quality group III nitride crystal by causing a reaction between a group V source material including nitrogen and a melt mixture of an alkali metal and a group III metal (Patent Reference 2).
Patent Reference 1                U.S. Pat. No. 5,868,837        
Patent Reference 2                Japanese Laid Open Patent Application 2001-58900        